The present invention relates to a magnetic disc device designed for ease of compatibility with other magnetic disc devices. Particularly, it relates to a system for processing track bytes of a magnetic disc device.
There is a widely known memory system in which a magnetic disc device is compatibly used with a different type of magnetic disc device having a different memory capacity or a different amount of data (bytes) stored in a track or cylinder without changing the program of the central processing unit (CPU), as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 52-28231 (corresponding to U.S. Pat. No. 4,223,390. A memory system of this type makes use of a record format which consists of a count portion, an error check code (ECC) therefor, a track byte portion, a key portion, an ECC therefor, a data portion, an ECC therefor, and gaps. As is well known, a magnetic disc device is usually provided with a pluraliy of magnetic discs, and the data is written in many concentric circles (tracks or cylinder) determined by position signals that are generated by a servo disc among the plurality of magnetic discs. Each cylinder has a starting point, and the position of the data stored in the cylinder is indicated by the distance (such as byte length or sector number) from the starting point. The data is packed in the above-mentioned format (record) and is recorded on the cylinder. Each cylinder has a large number of bytes so that many records can be written thereon.
In such a magnetic disc device, the recorded data is read in the order of the counter portion, the ECC thereof, the track byte, the key portion, the ECC thereof, etc. to check for errors. If an error exists in the key portion, for example, the process of reading out the recorded data is retried. The recorded data is read with the magnetic head of the disc deviating toward the right or left relative to the center line of the cylinder since the data is written with the magnetic head deviating toward the right or left relative to the center line of the cylinder due to thermal expansion or some other factor. Due to deviation of the magnetic head, the data can be correctly read from a portion where an error existed in the previous read operation. It is possible, however, that data may not be correctly read from the portions from which the data was correctly read in the previous read operation. But since a host disc control-device (DKC) stores correctly read data in an internal register, no problem arises even if the data is subsequently incorrectly read. However, in the conventional system, an adapter (ADP) and DKC do not store track byte information following the reading of the count portion, and the track bytes that were correctly read in the previous read operation disappear or are overwritten during retry. Therefore, if the track bytes are not correctly read at this time, incorrect information is stored in the ADP. A host unit generally does not transfer the track bytes and overlooks this fact. Hence, if the data is correctly read from the key portion, the data read from the data portion is received from the host unit. If the data portion is correct, the instructions for re-reading are terminated. Consequently, the ADP holds incorrect track bytes. When the recorded data is to be written in the next available region of the same track, the track bytes of the next region are prepared based upon the incorrect track bytes. The thus-prepared incorrect track bytes are then written in the track byte portion of the next track and the operation is repeated. Therefore, all of the track bytes of the subsequent records become erroneous, making it difficult to read the data.